[Clang importer/module printing] Correctly print NS_ERROR_ENUMs.
Ever since we stopped associating the top-level struct of an imported
NS_ERROR_ENUM with the Clang enum declaration, we've been unable to
print imported NS_ERROR_ENUMs. The module-printing infrastructure
would drop them thinking they aren't imported declarations.
This also affected NS_ERROR_ENUMs that were imported as members of
another type, as well as other types imported as members.
Fixes rdar://problem/32497693.
(cherry picked from commit 6a26f5f0446640aa1326cdef590294b5621d3153)
